U.S. Pat. No. 5,194,847 discloses “an apparatus for sensing intrusion into a predefined perimeter comprising means for producing a coherent pulsed light, which is injected into an optical sensing fiber having a first predetermined length and positioned along the predefined perimeter. A backscattered light in response to receiving the coherent light pulses is produced and coupled into an optical receiving fiber. The backscattered light is detected by a photodetector and a signal indicative of the backscattered light is produced. An intrusion is detectable from the produced signal as indicated by a change in the backscattered light. To increase the sensitivity of the apparatus, a reference fiber and an interferometer may also be employed.”
U.S. Pat. No. 5,778,114 discloses a fiber analysis system that “detects threats to a buried underground fiber by delivering two optical sub-signals, split from a single beam, into opposite ends of the fiber so the sub-signals traverse the fiber in opposite directions. The sub-signals are recombined into a beam whose characteristics are detected by a detector. The output signal of the detector is compared by a processor to different reference values stored in a data base representing different detector output signals corresponding to different fiber conditions. By matching the detector output signal to a reference value associated with a particular fiber condition, the processor can determine whether a potential threat exists. By inducing a sequence of known vibrations (tones) in the fiber at spaced locations along a generally orthogonal to the fiber, the processor can establish the location of the fiber by comparing the detector output signal to a reference signals corresponding to the known tones to determine which one was received in the shortest time
U.S. Patent Application Publication 20080088846 discloses an improved technique for acoustic sensing that involves, in one embodiment, “launching into a medium, a plurality of groups of pulse-modulated electromagnetic-waves. The frequency of electromagnetic waves in a pulse within a group differs from the frequency of the electromagnetic waves in another pulse within the group. The energy scattered by the medium is detected and, in one embodiment, may be used to determine a characteristic of the environment of the medium. For example, if the medium is a buried optical fiber into which light pulses have been launched in accordance with the invention, the presence of acoustic waves within the region of the buried fiber can be detected.”
U.S. Patent Application Publication 20080144016 discloses an improved technique for acoustic sensing that in one embodiment, involves “launching into a medium, a plurality of groups of pulse-modulated electromagnetic-waves. The frequency of electromagnetic waves in a pulse within a group differs from the frequency of the electromagnetic waves in another pulse within the group. The energy scattered by the medium is detected and, in one embodiment, the beat signal may be used to determine a characteristic of the environment of the medium. For example, if the medium is a buried optical fiber into which light pulses have been launched in accordance with the invention, the presence of acoustic waves within the region of the buried fiber can be detected.”
U.S. Pat. No. 4,477,725 discloses a system for remote measurement of structural forces that “includes a plurality of microbend transducers mounted along the length of the structure for microbending an optical fiber in response to structural forces, such as stress acting upon an oil or gas pipeline or the like. An optical time domain reflectometer (OTDR) has a light source for launching a pulsed optical signal for passage through the fiber and a photodetector for sensing as a function of time the intensity of backscattered light reflected back through the fiber, wherein this sensed time function is correlated directly with discrete longitudinal positions along the length of the fiber and the structure. When one or more of the microbend transducers is activated to induce a microbend in the fiber in response to localized forces acting upon the structure, a portion of the backscattered light is lost at the microbend. This attenuation in backscattered light intensity is sensed quantitatively and positionally identified by the photodetector.”
U.S. Pat. No. 6,449,400 discloses that “a sensing optical fiber can detect information with a high detection sensitivity by an OTDR method using Rayleigh scattered light. For this purpose, the sensing optical fiber includes a main line element which is an optical fiber installed as a light transmission line and sensor elements which are relatively short optical fibers that are inserted in intermediate parts of the main line element and whose core diameters are different from that of the main line element. The sensor system is capable of detecting various types of information obtained simultaneously with a high detection sensitivity by an OTDR method using Rayleigh scattered light. For this purpose, a sensor system includes sensing optical fibers and a measuring instrument which detects information around the light transmission line, in which the sensing optical fibers are provided, by using back-scattered light of the sensing optical fibers. In the sensor system, a plurality of the sensing optical fibers are provided, and the sensing optical fibers have a different number of the sensor elements, different distances between the sensor elements and different sensor element core diameters. Also, an optical switch, by which the sensing optical fibers are selectively switched, is provided between the sensing optical fibers and the measuring instrument.” The system “can be applied for the following purposes (1) Detection of distortion, fatigue, rupture or the like in structures such as general buildings, high-rise buildings, speedways, elevated bridges, tunnels, dams, flying fields, harbor facilities, and industrial facilities. (2) Detection of distortion, fatigue, rupture or the like in moving structures such as frames of airplanes, ship hulls, frames of motorcars, frames of railroad vehicles, and spacecraft bodies. (3) Detection of distortion, fatigue, rupture or the like in general building materials such as steel frames, wallplates, flooring, ceiling panels, and connecting fittings. (4) Detection of distortion, fatigue, rupture or the like in life-lines such as oil pipelines, oil storage tanks, gas pipelines, gas tanks, water pipelines, electric cables, and (maritime or underseas) communication cables. (5) Supervision for guarding against intruders into or protecting the environment of buildings, parks, and outside facilities. (6) Supervision of diastrophism and ground subsidence (for earthquake prediction). (7) Supervision or observation of living bodies. (8) Detection of liquid adhesion
U.S. Pat. No. 7,136,156 discloses “a method, system, and medium . . . for Optical Time Domain Reflectometer (OTDR) Data storage and retrieval in a networking environment. The method includes receiving an OTDR trace file in its native format that includes OTDR information, storing the trace file, receiving a request to present the OTDR information in a browser or some other interface, identifying a viewer that can render the OTDR information, and communicating the OTDR information to the browser. The system includes a user interface that facilitates uploading an OTDR trace file in its native format, a storage device for receiving the trace file, and a viewer that can render the data within the OTDR trace file.
In optical fiber surveillance systems the resolution and the sensitivity of the surveillance system is limited in directions perpendicular to the length of the fiber.